This invention relates to a method for determining basic values from a specimen of a dielectric material for analysis of its vulcanization characteristic and an apparatus for carrying out the method.
Polymers belong to the group of vulcanizable materials and the electric properties thereof have been the subject of relatively comprehensive studies and research for the major part of this century. These studies and research work have generated a great amount of knowledge about the interior structure of polymeric materials and how this is influenced by admixture of different additives such as plasticizers and organic or inorganic fillers. Dielectric measuring methods have also been used to a certain extent in studies of aging phenomena of polymeric materials. Controlling the vulcanizing or curing processes by means of the changes in the dielectric properties i.e. caused by cross-linking or curing reactions is not used much in practice and is nearly not used at all in rubber working technology.
At present the dielectric properties of different polymers are of interest in polymer working only because the dielectric losses can be utilized for generation of heat in connection with preheating or vulcanizing.
Despite the fact that the very earliest work in this special technique, below called vulcametry, was carried out as early as the end of the 1920's, the dielectric measuring methods have not yet had any real importance in vulcametry. This is partly because suitable measuring electrodes as well as directly recording measuring bridges have been lacking. However, the most obvious reason seems to be that the basic mechanisms about the influence of the crosslinking reactions on the dielectric relaxation phenomena are not yet known enough to be used in practical vulcametry.
As is well-known, a dielectric usually contains polarizable or polarized molecules or molecule groups having permanent or induced dipoles. In an electric field the dipoles are turned in the field direction and molecules containing permanent dipoles tend to orientate themselves in electric fields. How fast and to which extent this orientation takes place has to do with how the molecules interfere with each other. When a rubber material is vulcanized--crosslinked, a series of other side reactions except crosslinkages are formed in normal cases which are characteristic of each combination of rubber and vulcanizing agent. The formation and development of these reactions and reaction products can be followed by the aid of dielectric measuring methods. Thus, the dielectric vulcametry and consequently this invention are based on these changes in the polar properties of the vulcanized rubber.
A method developed about 1953 and described in U.S. Pat. No. 3,039,297 for continuous measurement of the crosslinking reaction in rubber mixtures is described in U.S. Pat. No. 3,039,297. This method can be said to be the start of modern vulcametry and is characterized in broad outline in that a continuous or periodic motion or force (tensile, compressive, shearing or torsional) is applied to a test specimen of unvulcanized rubber under simultaneous measurement of force and motion response, respectively. The force/movement is usually transferred to the test specimen by means of a rotor or a linearly movable paddle.
This method was accepted very rapidly and has become very popular which has generated a long series of different measuring apparatuses, which include "the Wallace-Shawbury Curometer", below called curometer, "the Cepar-Apparatus", "Viscurometer", "Vuremo", Zwick-Schwingelastometer" and that most known of all, viz. the so-called Monsanto-Rheometer, below called rheometer.
The original purpose of the technique here called vulcametry was quite simply to produce a functioning control method in the synthetical rubber industry rapidly growing in the post-war period. The vulcametry has thereafter also been found to be a very useful method for studying the reaction kinetics of the vulcanization process and has also been used for this purpose. However, in later years some criticism has been directed to this so-called traditional vulcametry which can be said to be mechanical. It has been shown that if the test specimen is heated relatively slowly and even after reaching temperature of equilibrium there are temperature gradients through the test specimen, that a non-desired sliding can arise between cavity and rotor and paddle, respectively, and that certain rubber materials have a tendency to become porous during the testing procedure.
Certain comparative studies with isothermal vulcanization which are considered to give acceptedly true values are described in Polymer Testing Vol. 1, No. 4, page 247, 1980 by R H Norman. It has also been shown that the rheometer gives a much longer vulcanization time than the curometer which, in turn, gives longer vulcanizatioin times than isothermal vulcanization. Examples of this are shown in the table below which indicates 90% of vulcanization time in seconds at different temperatures.
______________________________________ Temperature .degree.C. Isothermal vulc. Curometer Rheometer ______________________________________ 120 3 120 4 800 7 800 140 870 1 100 1 800 160 280 320 460 180 72 105 195 200 17 47 97 ______________________________________
The great differences at low temperatures apparent from the table are unexpected and are propably due to the fact that there is a considerable difference between the true average temperature of the test specimen and the measured temperature even after a very long period of time. On the other hand, the great difference at high temperatures is not directly unexpected. It has been shown that the rheometer in comparison with guaranteed isothermal conditions gives vulcanizing times that are about twice as long at most temperatures. There are two probable explanations of these substantial deviations. Firstly, heat is continuously lost as heat is diverted from the rotor via the rotor shaft to the drive unit of the rheometer, with the result that the rotor becomes colder than the rotor cavity. Due to this the average temperature of the rubber specimen will be considerably longer than the adjusted temperature and therefore the vulcanization process will proceed more slowly. Secondly, it will take a longer time to heat the specimen in a rheometer in comparison with the conditions prevailing when producing the isothermal results shown in the above table due to the fact that the test specimen in the rheometer is much thicker than 0.5 mm which is the thickness of the specimen used in the isothermal tests.
Another disadvantage of conventional vulcametry is the interpretation problems arising when data produced by the rheometer are to be used to determine vulcanization times of voluminous rubber products such as big rubber dampers, contract tires, mill linings etc. Therefore there is a great demand for a method or process enabling measurement of the vulcanization course directly during the vulcanization of the current products.